It is known to produce 3-dimensional objects having a complex shape using stereolithography, whereby the object is dissected into a plurality of contiguous laminates which are sequentially formed from a liquid medium which is polymerized upon being irradiated by a suitable source of illumination.
Thus, U.S. Pat. No. 4,575,330 (Hull) discloses a system for producing a three-dimensional object from a fluid medium capable of solidification when subjected to prescribed synergistic stimulation. The system creates a cross-sectional pattern of the object to be formed at a selected surface of a fluid medium which is capable of altering its physical state in response to appropriate synergistic stimulation by impinging radiation, particle bombardment or chemical reaction. Successive adjacent laminae, representing successive adjacent cross-sections of the object, are automatically formed and integrated together to provide a step-wise laminar buildup of the desired object.
Similarly, U.S. Pat. No. 4,801,477 (Fudim) describes a system for preparing three dimensional objects by irradiating uncured photopolymer by emitting radiation directly into desired areas within a quantity of the uncured photopolymer.
A radiation emitting surface within a quantity of uncured photopolymer is placed in contact with the area of the photopolymer to be irradiated, and radiation is then emitted to solidify the photopolymer to the desired extent. The radiation emitting surface may be coated with fluorinated ethylene propylene or ultra high molecular weight polyolefin. The positioning and irradiation steps can be repeated a desired number of times.
In known methods for forming 3-dimensional objects using stereolithography, a geometrical representation of the model is first produced and stored in a computer. Thereafter, the geometrical model is analyzed in order to produce laminates each having a uniform height and bound by upper and lower bases having equal dimensions. The side wall surface of each laminate is therefore perpendicular to the bases.
In such systems the heights of successive laminates are different, it clearly being desirable to maximize the height of each laminate in order to accelerate production of the complex shape, commensurate with preserving both the uniformity and the continuity of the object's external side wall surface.
In such an arrangement, the upper and lower bases of contiguous laminates are of different dimensions such that progressing from one laminate to the next results in a discontinuity in the curvature of the side wall surface of the object since, in effect, this is formed of a plurality of different sized plates formed contiguous with one another. Such discontinuities manifest themselves as "steps" in the side wall surface of the object and can be minimized by decreasing the height of each laminate. However, this increases the processing and manufacturing time, and hence the cost of the object. Furthermore, even by reducing the height of successive laminates, the step-wise side wall surface can never be completely cured of surface discontinuities.
Variations in the location of the radiation source with respect to the liquid medium being polymerized have been suggested in the art. Thus, for example, in above-mentioned U.S. Pat. No. 4,575,330 the liquid medium is illuminated from above whilst in U.S. Pat. No. 4,801,477 the illumination source is disposed within the liquid medium itself. U.S. Pat. No. 4,575,330 suggests an alternative embodiment wherein the liquid medium is contained within a transparent vessel which is illuminated through its base.
A. J. Herbert in "Solid Object Generation" appearing in the Journal of Applied Photographic Engineering, Vol. 8 No. 4, 1982, pp. 185-188 proposes a system employing a focused scanning laser beam whilst H. Kodama in "Automatic Method For Fabricating a 3-Dimensional Plastic Model With Photo-Hardening Polymer" appearing in Rev. Sci. Instrumentation Vol. 52 No. 11, 1981, pp 1770-1773 teaches the use of a space-modulated parallel light flux, the liquid medium being irradiated in both cases in accordance with the type of illumination.
Likewise, the location of the illumination source relative to the liquid medium is also influenced by the manner in which successive laminates are produced. In all cases, each successive laminate must be contiguous with the preceding laminate. However, this can be accomplished in one of two ways. According to one method, an upper surface of the liquid medium is illuminated and the resulting laminate is then lowered by a depth corresponding to the height of the next laminate, which is likewise polymerized, and so on. Alternatively, the solid object can be polymerized from beneath, so to speak, the object being withdrawn from the liquid medium as each successive laminate is polymerized.
In all cases, a geometrical or mathematical representation of the object is produced and stored in a CAD system. Successive laminates are then determined having optimized heights and bases of known contour, whereupon the laminates are formed by illuminating the liquid medium in a direction normal to a surface thereof.
As explained above, the side wall surface of each laminate is parallel to the direction of illumination, thereby producing a step-wise discontinuity in the external side wall surface of the object.